The present invention relates to an encoder including a measuring standard, a scanner for scanning the grating provided on the measuring standard and for generating a position signal, and a measuring standard carrier having a groove for receiving the measuring standard therein.
The encoder of this type are used for measuring a relative position of two parts and can be formed as linear or angular encoders.
German patent No. 2,505,587 discloses a linear encoder in which the measuring standard is attached to the carrier with a glue layer. In order to prevent measuring inaccuracies, resulting from different coefficients of thermal expansion of the carrier and the measuring standard, the attachment of the measuring standard to the carrier is effected without the measuring standard and the carrier directly contacting each other. The attachment is effected with an elastic glue. The drawback of such an attachment consists in that the measuring standard, because of being attached to the carrier only with an elastic glue layer, is not held sufficiently sturdy and is, therefore, susceptible co vibrations.
To eliminate this drawback, German patent 2,643,304 suggests to secure the measuring standard so that it would abut edges of a rectangular groove provided in the carrier and extending in the measuring direction. In the encoder of this German patent, the measuring standard in the middle portion thereof is secured in the groove with an unelastic glue and is secured with a highly elastic glue on both sides of the middle portion. The drawback of this solution consists in that the measuring standard, because of an elastic connection on only one surface, is still somewhat susceptible to vibrations. A direct contact of the measuring standard with the edges of the groove presents another drawback which consists in that the manufacturing errors of the carrier are transmitted to the measuring standard and results in measurement errors.
To reduce the vibrations of the measuring standard, it was suggested to secure a measuring standard in a carrier with more than one surfaces. Such an encoder is disclosed in German publication No. 3,118,607. In the encoder of DE-3,118,607, the measuring standard is secured in a longitudinal groove of the carrier, with the measuring standard directly contacting a side surface and the bottom of the groove. The space between the other side surface of the groove and the measuring standard is filled with a rubber bar and an elastic glue serving as a sealing compound. Thus, the measuring standard is rigidly secured in one region thereof. The drawback of this attachment consists in that the space, filled with the elastic glue, is relatively wide, which again adversely affect the vibration characteristics of the measuring standard. Another drawback of this attachment consists in that preloading of the measuring standard with a rubber bar prevents or substantially hinders free longitudinal expansion of the measuring standard and the carrier.
German Patent DE-3,605,789 discloses an encoder in which the measuring standard is likewise secured in a groove provided in a carrier. The measuring standard is embedded, along its narrow portion, in a permanently elastic sealing compound. To insure a longitudinal expansion of the measuring standard relative to the carrier, the sealing compound should be highly elastic and that adversely affects the vibration characteristics and the stability of the measuring standard.
Accordingly, an object of the invention is an encoder in which different thermal characteristics of the measuring standard and the carrier practically exercise no influence on the measurement results.
Another object of the invention is an encoder in which the measuring standard is secured to the carrier relatively vibration-free.